Various publications, including patents, published applications, technical articles, and scholarly articles, are cited throughout the specification. Each of these cited publications is incorporated by reference in this document, in its entirety and for all purposes.
Polymer resins, such as polyethylene terephthalate (PET), are widely used in the packaging industry. PET is a linear, thermoplastic polyester resin. The advantages of PET include toughness, clarity, good barrier properties, light weight, design flexibility, chemical resistance, and good shelf-life performance. Furthermore, PET is environmentally friendly because it can often be recycled. These characteristics of PET make it a popular material in the manufacturing of containers, for example, beverage bottles.
There are a variety of production methodologies to produce PET containers. For example, injection stretch blow molding is commonly used to make PET bottles. Of the various methodologies, one-piece PET containers having an integrated handle are commonly formed using extrusion blow molding (EBM). The EBM process includes extruding a polymer resin in a softened state through an annular die to form a molten hollow tube or parison. The molten parison is placed in a hollow blow mold having a cavity corresponding to the desired shape of the container being formed. Air is injected to inflate the parison against the interior walls of the blow mold. Upon contact with the walls, the parison cools rapidly and assumes the shape of the mold.
Polyesters are typically classified by inherent viscosity (I.V.) as a measure of molecular weight. To form beverage bottles, “bottle-grade” PET having an I.V. of about 0.72-0.84 dl/g is typically used. Bottle-grade PET has linear polymer chains and has a melt viscosity that is low enough to enable a fast injection stretch blow molding step with minimal resistance to flow. Bottle-grade PET generally cannot be used in the production of larger handle-ware containers using EBM, however, because of low melt strength. Melt strength is quantified by measuring melt viscosity at very low shear rates (approaching zero shear rate). Low melt strength hinders the ability to form a suitable parison. If a parison in the molten state has insufficient melt strength, the parison may form an hour-glass shape or may completely collapse as the parison is drawn down by its own weight, thereby resulting in the inability to produce a container. As melt strength increases, material distribution in the walls of the resultant container improves, and the process becomes more controllable and repeatable.
To make PET suitable for EBM, PET manufacturers have developed special grades of PET sometimes called extrusion PET or “EPET.” Typically, EPET is high molecular weight PET having an I.V. of 1.0 dl/g or greater as measured by solution viscosity. For PET resins, I.V. is used as a measure of molecular weight. The average molecular weight of a resin reflects the average length of polymer chains present in the resin. In general, melt strength increases with chain length and, thereby, also increases with molecular weight. Higher I.V. polymers generally require higher processing temperatures, however, which lead to certain processing challenges.
Given the higher temperatures at which EPET is melted and maintained during article manufacture, the molten EPET will degrade if production is halted for extended periods of time, which in turn will affect the quality of the containers produced from the degraded EPET. Degraded EPET is hotter, less viscous, tackier, and less predictable than other resins, particularly when used in upward extruding blow molding systems. Accordingly, degraded molten EPET should be removed from the molding apparatus and replaced with fresh molten EPET before restarting the molding run. The molten EPET to be removed from the blow molding apparatus also creates a hazard for workers tasked with removing the EPET material, and could damage mechanical or electrical components of molding systems if it comes in contact with them.
Safer and more efficient mechanisms for removing molten thermoplastic materials from upward extruding molding systems are needed, both to reduce the time needed to “reset” the apparatus by removing the old, degraded material and restarting the run, to protect workers who remove the old material from the molding system, and to protect the system components.
Typical mechanisms for redirecting flowing materials include the use of conventional valves, for example, an improved high-pressure ram valve as described in U.S. Pat. No. 4,867,197 issued to Ritter et al. Industrial valves are commercially available from suppliers such as SchuF (USA), Inc. of Mt. Pleasant, S.C. (www.schufusa.com). It is believed that conventional valves have not been configured for integration into an upward extruding blow molding flow head to direct molten thermoplastic materials from the flow head. Thus, there remains a need for an improved diverter valve for redirecting molten thermoplastic materials out of a blow molding flow head, with the valve configured for unified integration with the flow head.